Common mode feedback circuit with backgate control

ABSTRACT

A common mode feedback (CMFB) loop for a differential amplifier sense an output common mode of a differential circuit and provides feedback to the gates of tail current transistors. Many CMFB loops cannot easily adjust the output common mode voltage and the output common mode may vary over process, voltage, and temperature. An improved CMFB circuit adds a control circuit to control backgates of tail current transistor device(s) of the differential circuit such that the output common mode voltage can be made adjustable.

PRIORITY DATA

This patent application receives benefit from and/or claim priority toU.S. Provisional Patent Application Ser. No. 62/451,042, filed on Jan.26, 2017, entitled “PUSH PULL DIFFERENTIAL PAIR AND COMMON MODE FEEDBACKCIRCUIT”. This U.S. Provisional Patent Application is incorporated byreference in its entirety.

TECHNICAL FIELD OF THE DISCLOSURE

The present invention relates to the field of integrated circuits, inparticular to common mode feedback circuits with backgate control.

BACKGR0UND

Direct radio frequency communication systems and wide bandwidthinstrumentation systems are two important sources of motivation fordesigning extremely high speed analog-to-digital converters (ADCs). Inone example, such systems may require an ADC which can provide 12-bitresolution running at 10 Gigasamples/second. Certain process nodes canhelp make circuits in such high speed ADCs run faster, but can pose newchallenges for circuit designers. For instance, some process nodes mayrequire lower voltage designs. Limits on power consumption or generalpreference for lowering power consumption may also require lower voltagedesigns. At the same time, circuit designers aim to design circuitswhich can perform well.

One important part of an ADC are the amplifiers, which are used toamplify signals. One may find amplifiers connecting stages of a pipelineADC, or other ADC architectures. The amplifier is an important part ofthe signal chain in an ADC, and many other analog circuits. Performanceof the amplifier can directly affect many performance metrics of theoverall system.

To achieve certain benefits such as even order harmonics cancellations,differential or balanced circuits are often used to implementamplifiers. For differential circuits, including differential buffers,differential amplifiers, and push pull differential amplifiers, commonmode of a differential voltage may need to be adjusted for optimalperformance or requirements of the overall circuit. Common mode of adifferential voltage is a common voltage that appears in both signals,i.e., both the + and − parts of the differential voltage. Common modeadjustments or controls may be particularly useful when multiple stagesof differential circuits are cascaded one after another, and one stagemay impose common mode requirements on another stage. Common modefeedback (CMFB) circuits are used for adjusting/controlling common modeof differential voltages. Specifically, a CMFB circuit (sometimesreferred to as a CMFB loop) can be are added to differential circuits toadjust input common mode and/or output common mode of the differentialcircuits.

BRIEF DESCRIPTION OF THE DRAWINGS

To provide a more complete understanding of the present disclosure andfeatures and advantages thereof, reference is made to the followingdescription, taken in conjunction with the accompanying figures, whereinlike reference numerals represent like parts, in which:

FIG. 1 shows an exemplary differential amplifier having common modefeedback;

FIG. 2 shows another differential amplifier having common mode feedback;

FIG. 3 shows an exemplary differential amplifier having common modefeedback with backgate control, according to some embodiments of thedisclosure;

FIG. 4 shows another exemplary differential amplifier having common modefeedback with backgate control, according to some embodiments of thedisclosure;

FIG. 5 shows another exemplary differential amplifier having common modefeedback with backgate control, according to some embodiments of thedisclosure;

FIG. 6 shows yet another exemplary differential amplifier having commonmode feedback with backgate control, according to some embodiments ofthe disclosure; and

FIG. 7 shows a further exemplary differential amplifier having commonmode feedback with backgate control, according to some embodiments ofthe disclosure; and

FIG. 8 shows a flow diagram illustrating a method for common modefeedback and backgate control, according to some embodiments of thedisclosure.

DETAILED DESCRIPTION

A common mode feedback (CMFB) loop for a differential amplifier sensesan output common mode of a differential circuit and provides feedback tothe gates of tail current transistors. Many CMFB loops cannot easilyadjust the output common mode voltage and the output common mode mayvary over process, voltage, and temperature. An improved CMFB circuitadds a control circuit to control backgates of tail current transistordevice(s) of the differential circuit such that the output common modevoltage can be made adjustable.

According to some embodiments, an improved CMFB circuit is provided to adifferential amplifier, where the differential amplifier includes adifferential pair of transistors whose gates receive a differentialinput voltage and the drains of the transistors of the differential pairform the differential output nodes generating a differential outputvoltage. The differential amplifier further includes a pair of tailcurrent transistors (whose drains are connected to the sources ofrespective transistors of the differential pair). The tail currenttransistors are connected in negative feedback configuration, where thegates of the tail current transistors are coupled to the differentialoutput nodes. The output common mode of the CMFB circuit corresponds tothe gate to source voltage of the tail current transistors. Thebackgates of the tail current transistors are connected to or driven bya control circuit, such that the gate to source voltages can beadjustable. Accordingly the output common mode is also adjustable andcan track over process, voltage, and temperature.

In some embodiments, the CMFB circuit includes sensing circuitry tosense the output common mode and an operational amplifier (opamp) thatcan adjust/control the backgate voltages of tail current transistors todrive the sensed output common mode to match the target output commonmode. In one example, the CMFB circuit includes sense resistors coupledto the differential output nodes to generate a sensed output common modevoltage, and an opamp that receives the sensed output common modevoltage and a target common mode voltage. The opamp controls thebackgates of the tail current transistors to drive sensed output commonmode to match to the target common mode voltage. In another example, thesensing circuitry includes a replica bias circuit to sense a copy of theoutput common mode voltage, which is in turn used as the sensed commonmode voltage at the input of the opamp for driving the sensed commonmode voltage to match the target common mode voltage. In yet anotherexample, the opamp controls the backgate voltages of a tail currenttransistor in the main differential branches and a tail currenttransistor in the replica branch to drive the sensed output common modeto match the target output common mode.

Limitations of Some CMFB Circuits

FIG. 1 shows an exemplary differential amplifier having common modefeedback (CMFB). The differential amplifier as shown has a low swing.Such a low swing differential amplifier is often used as the input stageof a two stage amplifier. Transistors m0 and m1 form a differentialpair, referred herein as a differential pair of transistors. Thedifferential pair of transistors have gates that receive a differentialinput voltage. As shown, the gate of m0 receives the negative part V(−)of the differential input voltage and the gate of m1 receives thepositive part V(+) of the differential input voltage. Furthermore, thedifferential pair of transistors have drains that can form thedifferential output nodes of the differential amplifier. As shown inthis example, the drain of m0 and the drain of m1 form the differentialoutput nodes (indicated as +Vout−, or referred herein as the positivedifferential output node and the negative differential output node). Thedifferential pair of transistors m0 and m1 are in a common sourceconfiguration, or in other words, the sources of the differential pairof transistors are tied/coupled together. FIG. 7 shows a similarconfiguration, where the differential pair of transistors are cascoded.

Transistors m2 and m3 are the tail current transistors and can form apart of the CMFB circuit for the differential amplifier. The drains ofthe tail current transistors m2 and m3 are connected to respectivesources of the differential pair of transistors m0 and m1. The gates thetail current transistors m2 and m3 are coupled to the differentialoutput nodes. As shown, the gate of m2 is tied to the positivedifferential output node, and the gate of m3 is tied to the negativedifferential output node. The tail current transistors m2 and m3 are ina common source configuration, or in other words, the sources of m2 andm3 are tied/coupled together. The tail current transistors m2 and m3 arein a common drain configuration, or in other words, the drains of m2 andm3 are tied/coupled together. To the output common mode at thedifferential output nodes, the tail current transistors m2 and m3 areconnected in negative feedback and the output common mode is set toequal to the Vgs (gate to source voltage) of m2 and m3.

Further to the differential pair of transistors and the tail currenttransistors, the differential amplifier includes current sources I0 andI1 for supplying current for the respective transistors of thedifferential pair of transistors (m0 and m1 respectively).

One issue relating to the circuit seen in FIG. 1 is that the outputcommon mode being set by the Vgs of m2 and m3 can be quite limiting. TheVgs is determined by device parameters such as Vt (threshold voltage),W/L (width and length ratio) and the drain current. To achieve thedesired Vgs, i.e., the output common mode of the amplifier, a solutionmay require non-optimal sizing and drain current for m2 and m3. Inaddition, the variation of the Vgs over process, voltage, andtemperature (PVT) may be unacceptable since such variation can greatlyaffect the output common mode.

FIG. 2 shows another differential amplifier having common mode feedback.One solution for this Vgs limitation is to insert batteries or idealvoltage sources Vx 202 and Vx 204. With the batteries inserted betweenthe positive differential output node and the gate of m2 and between thenegative differential output node and the gate of m3, the output commonmode is equal to Vgs+Vx, Vx being a voltage supplied by a voltage sourceor battery. The battery can be made by a number of different circuittechniques. With the use of batteries as shown, it is possible toachieve the desired common mode while maintain some optimum sizing,drain current, and Vgs for m2 & m3. However, the output common modevoltage would still vary over PVT, which may not be optimal oracceptable in some scenarios. The output common mode voltage is noteasily adjusted unless Vx is made variable, which can complicate thecircuit design greatly.

Improved CMFB Circuits with Backgate Control

FIG. 3 shows an exemplary differential amplifier having common modefeedback with backgate control, according to some embodiments of thedisclosure. To provide more flexibility to the CMFB circuit, a controlcircuit 302 is included to control backgates of the tail currenttransistors m2 and m3. Adjusting the backgates of m2 and m3 can in turnadjust the Vt and the Vgs of m2 and m3. As explained previously withFIG. 1, the output common mode is set to the Vgs of m2 and m3. With theaddition of control circuit 302, the Vgs of m2 and m3 is adjustable bythe control circuit 302. The control circuit 302 can be designed to varythe Vgs of m2 and m3 over a fairly wide range allowing for an adjustablecommon mode voltage. The control circuit 302 can also be designed totrack over PVT. In some cases, one single control circuit 302 cancontrol both the backgates of m2 and m3. In other words, the tailcurrent transistors are controlled by a common output from the controlcircuit 302.

The control circuit 302 can include means for generating common modefeedback from differential output nodes of the differential amplifier,and control means coupled to backgates of tail current transistors forvarying an output common mode voltage of the differential amplifierbased on the common mode feedback. The means for varying the outputcommon mode voltage comprises means for driving the backgates of tailcurrent devices until the common mode feedback reaches a target outputcommon mode voltage. Exemplary implementations for such means areillustrated by FIGS. 4-6.

FIG. 4 shows another exemplary differential amplifier having common modefeedback with backgate control, according to some embodiments of thedisclosure. Specifically, FIG. 4 shows an embodiment where the controlcircuit 302 from FIG. 3 has an operational amplifier (opamp) A1 andcommon mode sense resistors R0 and R1. The output common mode voltageVCMout is sensed by R0 and R1. The opamp A1 generates an output voltageVbg, and drives the backgates of the tail current transistors m2 and m3until the sensed common mode voltage VCMout of the differential outputnodes reaches a target output common mode voltage VCM. In otherwords,the opamp A1 drives the backgates of m2 and m3 until VCMout=VCM.

In some cases, the resistors R0 and R1 may reduce the differential gainat the output, which may be undesirable in a high gain stage. Theresistors could be replaced by a switched capacitor circuit to addressthis issue. In some cases, a replica bias could be used to sense thecommon mode instead (which is shown in FIGS. 5 and 6). Referring back toFIG. 4, the control circuit can be designed to track over PVT whilemaintaining the output common mode constant and equal to VCM, as long asthere is sufficient gain from backgates of m2 and m3 to VCMout and theswing of opamp A1 is capable of going below ground.

FIG. 5 shows another exemplary differential amplifier having common modefeedback with backgate control, according to some embodiments of thedisclosure. The CMFB circuit includes a replica bias circuit thatprovides the sensed common mode voltage VCMsense. The replica biascircuit senses a copy of the output common mode voltage of thedifferential output nodes. Specifically, the replica bias circuitreplicates at least a part of the differential amplifier. In thisexample, the replica bias circuit has transistor m4, tail currenttransistor m5, and current source I2. Transistor m4, tail currenttransistor m5, and current source I2 are sized to run at the samecurrent density as the devices in the differential amplifier (i.e., m0,m2, I0, and m1, m3, and I1). Transistor m4's gate can be driven by aninput common mode voltage VCMin. Opamp A1 servoes/drives the backgatesof tail current transistors m2, m3, and m5 with Vbg until VCMsense=VCM.VCMout (the output common mode) would also equal VCM as long as m2, m3,and m5 are matched and at the same current density.

The circuit seen in FIG. 5 can have some advantages over the circuit ofFIG. 4. First, since there are no resistors (e.g., R0 and R1) sensingthe common mode in FIG. 5, the differential amplifier can be designedfor higher gain. Second, the common mode sense circuit is separate fromthe signal path and can thus be immune to large signal glitches. Third,it can also be low power by scaling.

This circuit in FIG. 5 can be designed to track over PVT whilemaintaining the output common mode constant and equal to VCM as long asthe devices match, and there is sufficient gain from backgate toVCMsense, and the opamp swing is capable of going below ground.

FIG. 6 shows yet another exemplary differential amplifier having commonmode feedback with backgate control, according to some embodiments ofthe disclosure. For some cases, a wide swing is required or desirablewith relatively low gain. As shown in FIG. 6, resistors R0 and R1 areused to sense the output common mode VCMout, and VCMout drives the(front) gate of tail current transistor m2. The control circuit hasopamp A1 and replica bias (which includes current source I2, transistorm4, and tail current transistor m5). VCM is a target common modevoltage. The control circuit, i.e., the opamp A1 provides Vbg and drivesthe backgates of tail current transistor m2 and replica tail currenttransistor m5 until the Vgs of m5 (which is node VCMsense) reaches VCM.If transistor m2 and transistor m5 are matched and at equal currentdensity, and transistor m0, transistor m1, and transistor m4 are matchedand at equal current density, then the output common mode voltage VCMoutwould also equal VCM.

FIG. 7 shows a further exemplary differential amplifier having commonmode feedback with backgate control, according to some embodiments ofthe disclosure. In some embodiments where there is sufficient headroom,the differential pair of transistors are cascoded or actively cascoded.As seen in FIG. 7, cascode transistor m6 and cascode transistor m7 areadded to m0 and m1 respectively. In other words, the differential pairof transistors are cascoded. The gates of m6 and m7 can be driven byappropriate bias voltages Vb1 and Vb2 (which may be different voltagesor the same voltages) respectively to ensure proper operation. Sourcesof m6 and m7 are connected to the respective drains of m0 and m1. Drainof m6 and the drain of m7 form the differential output nodes (indicatedas +Vout−, or referred herein as the positive differential output nodeand the negative differential output node). The gates of m2 and m3 areconnected to the respective drains of m6 and m7. The control circuit 302can include circuitry which senses the output common mode voltage at thedrains of m6 and m7. Adding the cascode transistors m6 and m7 canimprove performance of the differential amplifier. The functionality ofbackgate control is the same or similar to the embodiments illustratedby FIGS. 3-6.

With reference to FIG. 4, the common mode sense resistors R0 and R1 cansense the output common mode voltage VCMout at the differential outputnodes. In the case of FIG. 7, when the differential pair of transistorsare cascoded as seen in FIG. 7, the differential output nodes are at thedrains of m6 and m7 (the cascode transistors), and R0 and R1 would beconnected to respective drains of m6 and m7.

With reference to FIGS. 5 and 6, the replica bias circuit replicates atleast a part of the differential amplifier. In the case of FIG. 7, whenthe differential pair of transistors are cascoded as seen in FIG. 7, thereplica bias circuit would also replicate the cascode transistor. Forinstance, the replica bias circuit can replicate I0, m6, mo, and m2, orreplicate I1, m7, m1, and m3.

Exemplary Method for CMFB with Backgate Control

FIG. 8 shows a flow diagram illustrating a method for common modefeedback and backgate control for a differential amplifier, according tosome embodiments of the disclosure. In 802, common mode feedback ofdifferential output nodes of the differential amplifier is received. Inother words, feedback of the output common mode is provided to a controlcircuit. For some cases, the common mode feedback is generated by senseresistors coupled to the differential output nodes. For some cases, thecommon mode feedback is generated by a replica bias circuit sensing acopy of an output common mode voltage of the differential output nodes.In 804, a control circuit drives one or more backgates of one or moretail current transistors of the differential amplifier based on thecommon mode feedback and a target output common mode voltage. In somecases, the control circuit can drive driving a backgate of a furthertail current transistor in the replica bias circuit based on the commonmode feedback and the target output common mode voltage.

In 804, driving the one or more backgates can include varying a voltageat the one or more backgates until the common mode feedback reaches thetarget output common mode voltage. In some embodiments, driving the oneor more backgates can include changing gate to source voltages and/orthreshold voltages of the tail current transistors. In some embodiments,driving the one or more backgates can include varying gate to sourcevoltages and/or threshold voltages of the tail current transistors overone or more of the following: power, voltage, and temperature. As aresult, the backgate control provides a variable output common modevoltage for the differential amplifier. The control circuit can bedesigned to track over PVT while maintaining the output common modeconstant and equal to the target output common mode voltage.

In some embodiments, the method includes receiving a differential inputvoltage at a differential pair of transistors of the differentialamplifier, wherein drains of the differential pair of transistors formthe differential output nodes of the differential amplifier. Thedifferential pair of transistors are seen as m0 and m1 in the FIGURES.In some embodiments, the one or more gates of the one or more tailcurrent transistors are coupled to the differential output nodes. Inother words, the tail current transistors are in negative feedbackconfiguration, as illustrated in the FIGURES. In some embodiments, theone or more gates of the one or more tail current transistors are drivenby the common mode feedback (e.g., as seen in FIG. 6).

EXAMPLES

Example 1 is a differential amplifier having common mode feedback andbackgate control, comprising: differential pair of transistors whosegates receive a differential input voltage, tail current transistorswhose drains are connected to respective sources of the differentialpair of transistors and gates are coupled to differential output nodesof the differential amplifier, and a control circuit controllingbackgates of the tail current transistors. In some cases, the drains ofthe differential pair of transistors can form the differential outputnodes of the differential amplifier. In some cases, the differentialpair of transistors are cascoded, thus the drains of cascode transistorsform the differential output nodes of the differential amplifier. Insome cases, a single tail current transistor is provided (see FIG. 6).

In Example 2, the differential amplifier of Example 1 can furtherinclude current sources supplying current for the respective transistorsof the differential pair of transistors.

In Example 3, the differential amplifier of Example 1 or 2 can furtherinclude the differential pair of transistors having sources which arecoupled together.

In Example 4, the differential amplifier of any one of Examples 1-3 canfurther include the tail current transistors having sources which arecoupled together.

In Example 5, the differential amplifier of any one of Examples 1-4 canfurther include the tail current transistors being controlled by acommon output from the control circuit.

In Example 6, the differential amplifier of any one of Examples 1-5 canfurther include the control circuit comprising: an operational amplifierdriving the backgates of the tail current transistors until a sensedcommon mode voltage of the differential output nodes reaches a targetoutput common mode voltage.

In Example 7, the differential amplifier of Example 6 can furtherinclude the sensed common mode voltage is provided by a replica biascircuit sensing a copy of an output common mode voltage of thedifferential output nodes.

Example 8 is a method for common mode feedback and backgate control fora differential amplifier, the method comprising: receiving common modefeedback of differential output nodes of the differential amplifier; anddriving one or more backgates of one or more tail current transistors ofthe differential amplifier based on the common mode feedback and atarget output common mode voltage.

In Example 9, the method in Example 8 can further include generating thecommon mode feedback by sense resistors coupled to the differentialoutput nodes.

In Example 10, the method in Example 8 or 9 can further includegenerating the common mode feedback by a replica bias circuit sensing acopy of an output common mode voltage of the differential output nodes.

In Example 11, the method in any one of Examples 8-10 can furtherinclude driving a backgate of a further tail current transistor in areplica bias circuit based on the common mode feedback and the targetoutput common mode voltage.

In Example 12, the method in any one of Examples 8-11 can furtherinclude driving the one or more backgates comprising: varying a voltageat the one or more backgates until the common mode feedback reaches thetarget output common mode voltage.

In Example 13, the method in any one of Examples 8-12 can furtherinclude driving the one or more backgates comprising: changing gate tosource voltages and/or threshold voltages of the tail currenttransistors.

In Example 14, the method in any one of Examples 8-13 can furtherinclude driving the one or more backgates comprising: varying gate tosource voltages and/or threshold voltages of the tail currenttransistors over one or more of the following: power, voltage, andtemperature.

In Example 15, the method in any one of Examples 8-14 can furtherinclude receiving a differential input voltage at a differential pair oftransistors of the differential amplifier, wherein drains of thedifferential pair of transistors form the differential output nodes ofthe differential amplifier.

In Example 16, the method in any one of Examples 8-15 can furtherinclude the one or more tail current transistors having gate(s) whichare coupled to the differential output nodes.

In Example 17, the method in any one of Examples 8-16 can furtherinclude the one or more tail current transistors having gate(s) whichare driven by the common mode feedback.

Example 18 is an apparatus comprising: a differential amplifier; meansfor generating common mode feedback from differential output nodes ofthe differential amplifier; and control means coupled to backgates oftail current transistors for varying an output common mode voltage ofthe differential amplifier based on the common mode feedback.

In Example 19, the apparatus of Example 18 can further include thecontrol means for varying the output common mode voltage comprisingmeans for driving the backgates of tail current devices until the commonmode feedback reaches a target output common mode voltage.

In Example 20, the apparatus of Example 18 or 19 can further include themeans for generating the common mode feedback comprising circuitryreplicating at least a part of the differential amplifier.

Example 21 is an apparatus comprising means for implementing and/orcarrying out any one of the methods in Examples 8-17.

Variations, Applications, and Implementations

ADCs can be found in many places such as broadband communicationsystems, audio systems, receiver systems, etc. ADCs can translate analogelectrical signals representing real-world phenomenon, e.g., light,sound, temperature or pressure for data processing purposes. Designingan ADC is a non-trivial task because each application may have differentneeds in performance, power, cost and size. ADCs are used in a broadrange of applications including communications, energy, healthcare,instrumentation and measurement, motor and power control, industrialautomation and aerospace/defense.

Note that the activities discussed above with reference to the FIGURESof the present disclosure can be applicable to any integrated circuitsthat are used for data conversion. For instance, ADCs havingdifferential amplifiers like the ones illustrated herein can benefitfrom the common mode feedback circuits described herein. In certaincontexts, the features discussed herein can be applicable to medicalsystems, scientific instrumentation, wireless communications, and wiredcommunications, radar, industrial process control, audio and videoequipment, current sensing, instrumentation, etc.

In the discussions of the embodiments of the present disclosure, thecapacitors, clocks, DFFs, dividers, inductors, resistors, amplifiers,switches, digital core, transistors, and/or other components can readilybe replaced, substituted, or otherwise modified in order to accommodateparticular circuitry needs. Moreover, it should be noted that the use ofcomplementary electronic devices, hardware, software, etc. offer anequally viable option for implementing the teachings of the presentdisclosure. It is understood by one skilled in the art that a transistordevice can be generalized as a device having three main terminals:drain, source, and gate. Drain and source can be considered as aninput/output terminals, and gate can be considered as a controlterminal. A transistor device can also have a backgate. A backgate canbe considered a control terminal.

It is also imperative to note that all of the specifications,dimensions, and relationships outlined herein (e.g., the number ofdevices, etc.) have only been offered for purposes of example andteaching only. Such information may be varied considerably withoutdeparting from the spirit of the present disclosure, or the scope of theappended claims. The specifications apply only to one non-limitingexample and, accordingly, they should be construed as such. In theforegoing description, example embodiments have been described withreference to particular processor and/or component arrangements. Variousmodifications and changes may be made to such embodiments withoutdeparting from the scope of the appended claims. The description anddrawings are, accordingly, to be regarded in an illustrative rather thanin a restrictive sense.

Note that with the numerous examples provided herein, interaction may bedescribed in terms of two, three, four, or more electrical components.However, this has been done for purposes of clarity and example only. Itshould be appreciated that the system can be consolidated in anysuitable manner. Along similar design alternatives, any of theillustrated components, modules, and elements of the FIGURES may becombined in various possible configurations, all of which are clearlywithin the broad scope of this Specification. In certain cases, it maybe easier to describe one or more of the functionalities of a given setof flows by only referencing a limited number of electrical elements. Itshould be appreciated that the electrical circuits of the FIGURES andits teachings are readily scalable and can accommodate a large number ofcomponents, as well as more complicated/sophisticated arrangements andconfigurations. Accordingly, the examples provided should not limit thescope or inhibit the broad teachings of the electrical circuits aspotentially applied to a myriad of other architectures.

Note that in this Specification, references to various features (e.g.,elements, structures, modules, components, steps, operations,characteristics, etc.) included in “one embodiment”, “exampleembodiment”, “an embodiment”, “another embodiment”, “some embodiments”,“various embodiments”, “other embodiments”, “alternative embodiment”,and the like are intended to mean that any such features are included inone or more embodiments of the present disclosure, but may or may notnecessarily be combined in the same embodiments.

It is also important to note that the functions described herein, e.g.,functions relating to FIG. 8, illustrate only some of the possiblefunctions that may be carried out by the circuits illustrated in theFIGURES. Some of these operations may be deleted or removed whereappropriate, or these operations may be modified or changed considerablywithout departing from the scope of the present disclosure. In addition,the timing of these operations may be altered considerably. Thepreceding operational flows have been offered for purposes of exampleand discussion. Substantial flexibility is provided by embodimentsdescribed herein in that any suitable arrangements, chronologies,configurations, and timing mechanisms may be provided without departingfrom the teachings of the present disclosure.

Numerous other changes, substitutions, variations, alterations, andmodifications may be ascertained to one skilled in the art and it isintended that the present disclosure encompass all such changes,substitutions, variations, alterations, and modifications as fallingwithin the scope of the appended claims. Note that all optional featuresof the apparatus described above may also be implemented with respect tothe method or process described herein and specifics in the examples maybe used anywhere in one or more embodiments.

What is claimed is:
 1. A differential amplifier having common modefeedback and backgate control, comprising: differential pair oftransistors whose gates receive a differential input voltage; tailcurrent transistors whose drains are connected to respective sources ofthe differential pair of transistors and gates are coupled todifferential output nodes of the differential amplifier; and a controlcircuit controlling backgates of the tail current transistors.
 2. Thedifferential amplifier of claim 1, further comprises: current sourcessupplying current for the respective transistors of the differentialpair of transistors.
 3. The differential amplifier of claim 1, whereinthe differential pair of transistors have sources which are coupledtogether.
 4. The differential amplifier of claim 1, wherein the tailcurrent transistors have sources which are coupled together.
 5. Thedifferential amplifier of claim 1, wherein the tail current transistorsare controlled by a common output from the control circuit.
 6. Thedifferential amplifier of claim 1, wherein the control circuitcomprises: an operational amplifier driving the backgates of the tailcurrent transistors until a sensed common mode voltage of thedifferential output nodes reaches a target output common mode voltage.7. The differential amplifier of claim 6, wherein the sensed common modevoltage is provided by a replica bias circuit sensing a copy of anoutput common mode voltage of the differential output nodes.
 8. A methodfor common mode feedback and backgate control for a differentialamplifier, the method comprising: receiving common mode feedback ofdifferential output nodes of the differential amplifier; and driving oneor more backgates of one or more tail current transistors of thedifferential amplifier based on the common mode feedback and a targetoutput common mode voltage.
 9. The method of claim 8, furthercomprising: generating the common mode feedback by sense resistorscoupled to the differential output nodes.
 10. The method of claim 8,further comprising: generating the common mode feedback by a replicabias circuit sensing a copy of an output common mode voltage of thedifferential output nodes.
 11. The method of claim 8, furthercomprising: driving a backgate of a further tail current transistor in areplica bias circuit based on the common mode feedback and the targetoutput common mode voltage.
 12. The method of claim 8, wherein drivingthe one or more backgates comprises: varying a voltage at the one ormore backgates until the common mode feedback reaches the target outputcommon mode voltage.
 13. The method of claim 8, wherein driving the oneor more backgates comprises: changing gate to source voltages and/orthreshold voltages of the tail current transistors.
 14. The method ofclaim 8, wherein driving the one or more backgates comprises: varyinggate to source voltages and/or threshold voltages of the tail currenttransistors over one or more of the following: power, voltage, andtemperature.
 15. The method of claim 8, further comprising: receiving adifferential input voltage at a differential pair of transistors of thedifferential amplifier, wherein drains of the differential pair oftransistors form the differential output nodes of the differentialamplifier.
 16. The method of claim 8, wherein the one or more tailcurrent transistors have gate(s) which are coupled to the differentialoutput nodes.
 17. The method of claim 8, wherein the one or more tailcurrent transistors have gate(s) which are driven by the common modefeedback.
 18. An apparatus comprising: a differential amplifier; meansfor generating common mode feedback from differential output nodes ofthe differential amplifier; and control means coupled to backgates oftail current transistors for varying an output common mode voltage ofthe differential amplifier based on the common mode feedback.
 19. Theapparatus of claim 18, wherein the control means for varying the outputcommon mode voltage comprises means for driving the backgates of tailcurrent devices until the common mode feedback reaches a target outputcommon mode voltage.
 20. The apparatus of claim 18, wherein the meansfor generating the common mode feedback comprises circuitry replicatingat least a part of the differential amplifier.